How to Reanimate Dead Spots in Wi-Fi Networks

Thursday Jul 7th 2011 by Gerry Blackwell

We look at various options to remedy localized Wi-Fi network outages.

It’s the bane of existence for wireless network designers, administrators and home owners with wireless networks, too: the dreaded dead spot or coverage hole. Everywhere else in your home or office, you get excellent Wi-Fi coverage, but in one or two places, coverage inexplicably disappears or is very poor.

What’s going on?

It may be a simple matter of radio range. The dead spot is at or near the furthest extent of the access point’s coverage range.

Access points installed inside homes or offices never achieve the maximum range their manufacturers claim because effective range is reduced by obstacle interference from such things as walls, ceilings and other wireless networks and devices.

Even within the effective range of an access point, a strong interferer such as a close neighbor’s wireless network using the same channel as yours can cause small localized dead spots.

Most perplexing, though, are situations where connectivity is acceptable at some times but not others. The culprit in those situations, according to Jacob Sharony, principal consultant and president of Mobius Consulting, a wireless networking consultancy, is most likely multipath interference.

Radio signals transmitted indoors split and follow multiple paths to their destination, as they bounce off or pass through walls and other obstacles in different ways and at different angles.

The result is that multiple signals, some weaker, some stronger, arrive at the destination at different times -- and interfere with each other, sometimes enough to cause outages or unacceptably poor connectivity.

“Multipath is pretty much random,” Sharony said, which makes it virtually impossible to predict. For example, a door that's closed rather than open or a person sitting in a chair or not sitting in a chair can potentially alter multipath effects.

Extending the Range of Wi-Fi

If you discover a dead spot in a room or office where there is only one device that needs to connect to the network, you may be able to solve the problem by adding a range-extending adapter such as the Wi-Fire USB adapter (about $50) from hField, that claims to boost range up to three times.

It’s not a particularly elegant solution, and it only works in 802.11 b and g mode -- you won’t get the higher speeds of the latest 802.11n generation of Wi-Fi equipment.

Instructables, a website for do-it-yourself geeks, suggests you can even build your own range-extending adapter by plugging a USB stick adapter -- even an 11n adapter -- into an extension cable (to move it closer to the AP) and then mount it in a signal-concentrating dish such as a metal strainer/steamer. (Note: this is not a recommendation -- we have no idea if this actually works.)

The good news about 802.11n technology is that it reduces the likelihood of dead spots occurring in the first place -- though it does not eliminate them entirely.

The MIMO (multiple input, multiple output) technology built into 11n APs and adapters uses multiple antennas and creates optimal links between AP and client, responding on the fly to changing network conditions and multipath characteristics.

An 802.11n access point connecting to earlier-generation Wi-Fi devices will get some benefit from this "beam steering" capability in the new technology, Sharony says, but you only get the full benefits when 11n client devices are connecting to an 11n AP.  

If you discover dead spots in a small Wi-Fi network, even when using 11n gear, you may be able to eliminate them simply by moving the single router or AP to a more central position, or moving it away from interfering obstacles, or up higher.

The difficulty often is extending Ethernet connectivity to the new location. If it can’t be done, the next simplest solution is a range extender.

Some SOHO network vendors sell simple wireless range extenders or repeaters that you position between the router and the coverage hole. They receive signals from the router and retransmit them at a higher power level into the dead spot.

Most only work with selected routers and APs from the same vendor. However, Netgear says its Universal WiFi Range Extender WN2000RPT (about $75) will work with any AP. Linksys also has a range extender/bridge product, the RE1000 ($79), coming soon.

These are all Band-Aid solutions, though. The best practice, Sharony says, is to conduct a site survey to identify potential dead spots and design a network that eliminates them from the get-go.

This is one of the important functions that wireless consultants like Sharony can perform. Network vendors and integrators can also do it for you.

One key when designing Wi-Fi networks, Sharony says, is to always build in a “cushion” of radio signal strength, a kind of margin of error, so that when multipath interference causes a dip in strength, as it inevitably will, the signal will remain strong enough to provide adequate connectivity everywhere in the facility.

Invest in a Wi-Fi Site Survey

To conduct a site survey, a consultant sets up an AP in the environment and connects to it using a laptop computer running special mapping software, preloaded with a plan of the facility.

He then moves around the building, using the software to map coverage characteristics -- where signals are weaker, where they’re stronger.

“What you end up with is like a heat map, showing maybe green for good [connectivity], yellow for borderline, red for poor, for every place in the office,” Sharony says.

The best of this type of software is smart enough to automatically calculate the number of access points needed to cover a facility or floor and make recommendations for their optimal placement.

Sharony’s firm uses Site Survey software from Ekahau, a Finnish firm. It’s a product designed for network professionals and costs about $5,000, he says. Another comparable product often used by pros is AirMagnet Survey from Fluke Networks.

But Ekahau also has a less capable freeware version called Heat Mapper. There are other freeware, evaluation and inexpensive site survey products, including VisiWave Site Survey SO from AZO Technologies and Covera Zone from Celtrio.

If you conduct a site survey and discover you need more than one AP to eliminate all potential dead spots, there are a few possible ways to deploy and configure them. In enterprise networks, each AP is typically connected to an Ethernet "backbone" that connects them to the Internet.

More and more new homes today are wired with Ethernet in every room, so it may be possible to design a network in a large new home the same way. But in most homes and many small offices, running Ethernet to all APs will be difficult or impossible.

In that event, you will have to either resort to some of the Band-Aid solutions mentioned earlier or install multiple APs capable of working in "bridge" or "repeater" mode -- in effect creating a wireless backbone. This is also referred to as a Wireless Distribution System (WDS).

One router or AP, connected to the Internet, functions as the root, while others, configured in bridge mode and located nearer the dead spots, connect wirelessly to the root, and at the same time, provide wireless connectivity to nearby client devices.

Even if you assume you’ll only need one AP and don’t want to go to the trouble or expense of conducting a site survey, it makes sense to select a router or AP that can support bridge mode. Most enterprise products do, but only newer and higher-end SOHO products do.

If you start with a SOHO device that cannot support bridge mode and later discover you need additional APs, you will have to abandon the first product and start over (or use Band-Aid solutions).

Also be aware that for all practical purposes you will have to standardize on one vendor’s products and possibly one model, because bridge-mode functionality typically uses proprietary protocols and will not work between products from different vendors, or sometimes even between different models from the same vendor.

Some APs can only function in point-to-point bridge mode, meaning they can only connect wirelessly to one other AP. Some support point-to-multipoint mode, connecting in bridge mode to multiple other APs.

With products supporting point-to-multipoint bridging, mostly enterprise products, you can create a wireless mesh, making it even easier to eliminate dead spots caused by transient multipath effects. Mesh networks have many potential routes for sending traffic and can pick the route with the best throughput at any given moment.

Linksys, Cisco’s consumer brand, does not include WDS functionality in its router products on the assumption that it’s not required in a residence. It’s Cisco-branded small business routers do have this feature.

Netgear says all of its current consumer and SOHO grade routers support repeater mode or WDS in both point-to-point and point-to-multi-point modes.

Coverage holes or dead spots are an almost inevitable problem with even the simplest Wi-Fi networks. They can be filled in, however. The best approach is a site survey and a network designed to eliminate dead spots at the outset.

In the absence of a site survey, if you’re lucky, you can eliminate dead spots with minimal expense and effort -- but count on having to install additional infrastructure.

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